The transition metal catalyzed addition of silicone hydrides to olefins is well known. The application of this reaction to the curing of polyolefinic rubbers for the preparation of thermoplastic elastomers by dynamic vulcanization is also well known. The bulk of the commercially available thermoplastic vulcanizates are produced by the dynamic vulcanization of EPDM (copolymer of ethylene, propylene, and a limited amount of a diene such as 5-ethylidene-2-norbornene) rubber in isotactic polypropylene.
In the dynamic vulcanization process, the thermoplastic, rubber, and paraffinic oil (which is miscible in all proportions with rubber and molten plastic) are intensively sheared in an extruder, above the melting temperature of the plastic, to form an intimate melt blend of the polymeric materials. The rubber is then selectively cured, without affecting the plastic phase, while shearing of the polymer melt blend is continued.
The high rubber content formulations that are necessary for the preparation of soft thermoplastic vulcanizates, have a rubber continuous/discreet phase morphology on melt mixing. On curing, the continuous rubber phase breaks up and disperses into a continuous molten thermoplastic phase as crosslinked particulate rubber. The molten TPV thus formed consists of oil-swollen crosslinked rubber particles that are contained in an oil solution of molten thermoplastic, polypropylene in a particular embodiment. On cooling, the melt solidifies by the crystallization of the polypropylene, and the oil rejected by the polypropylene crystals is absorbed into the amorphous polypropylene phase and into the particulate rubber. The final product morphology consists of oil-swollen, crosslinked particulate rubber that is contained in an oil-swollen continuous polypropylene matrix. The oil present reduces TPV melt viscosity, which is critical in facilitating TPV manufacture, fabricability, and in producing fabricated parts with a smooth surface, in addition to reducing product raw material cost. The presence of clay, talc or other such solid filler in the TPV formulation prevents blocking of the granulated rubber, and thus allows accurate metering of the clay and rubber blend into the extruder. Inclusion of clay in the TPV also reduces product raw material cost.
It is well known that silicone hydrides are the curatives of choice when EPDM rubber is dynamically vulcanized by hydrosilylation in polypropylene (U.S. Pat. No. 4,803,244), because these curatives allow a high enough rubber cure rate that causes the rubber to be essentially fully cured in about the 40-second cure time that is available in the process (U.S. Pat. No. 4,594,390).
It was later recognized that the sterically unhindered vinyl groups that are pendent to the rubber backbone in the copolymer of ethylene, propylene, and a limited amount of 5-vinyl-2-norbornene (i.e., EP(VNB)DM), are much more reactive in hydrosilylation than the pendent ethylidene groups of EP(ENB)DM) rubber (U.S. Pat. No. 5,672,660). Thus the dynamic vulcanization of EP(VNB)DM in polypropylene by hydrosilylation could be achieved using a very low catalyst level (<20 ppm Pt°, based on the rubber content), in comparison with the case when EP(ENB)DM is dynamically vulcanized.
The preferred class of silicone hydride curatives described in U.S. Pat. No. 5,672,660 and in U.S. Pat. No. 6,150,464 is trimethylsilyl-terminated polymethylhydrosiloxanes, particularly when alkylated, as depicted in equation (1).
wherein R″ in structure 2 is the CH2═CHR group. In equation (1), the structure (1) refers to the silicone hydride prior to undergoing the alkylation reaction, and the structure (2) is the “alkylated” silicone hydride. When R″ is a C4 or greater, it is typically more rubber-reactive, as it's solubility in the rubber improves. The alkyl groups are chosen so as to allow miscibility of the rubber with the curative, which allows increased curing efficiency, provided the alkylation does not significantly reduce the active hydrogen atoms that are directly bound to the silicon atoms. The alkylation reaction itself (equation 1) is typically a platinum catalyzed hydrosilylation of an olefin with the silicone hydride. The alkylation reaction increases curative cost.
Colorable, non-hygroscopic TPVs can be obtained by the hydrosilylation cure of EP(VNB)DM, when dynamically vulcanized in polypropylene. Products with increased whiteness (high color “L” value) and color control during manufacture are highly desirable. The presence of zinc oxide and clay in the TPV allows a high color L value. Variations in process conditions such as above-normal temperatures can cause a decrease in product color L. In this invention, it has been discovered, unexpectedly, that product color L is sensitive to the injection location (along the extruder) of the silicone hydride curative and of the platinum catalyst.
Also, the inventors have found that unalkylated silicone hydride curatives (as in FIG. 2, structure 1) yield TPVs with increased color L over TPVs produced with alkylated silicone hydrides (such as in equation 1, structure 2). Furthermore, the unalkylated silicone hydrides, although immiscible with EP(VNB)DM rubber, have been found to be as efficient, if not modestly better TPV curatives, on a weight basis, than the rubber-miscible alkylated silicone hydrides. That is, the unalkylated silicone hydrides allow the preparation of TPVs with comparable, if not improved elastic recovery at 100° C. with other TPV physical properties, processability, and fabricated part appearance that match those of TPVs prepared with the alkylated silicone hydrides. These findings are contrary to the teachings in the art.